Advanced ceramic materials having applications as infrared (IR) or radio frequency (RF) transmitting window materials are being developed to meet the increasing performance requirements of strategic and battlefield missile systems. Ceramic materials which have been identified for window and radome applications include zinc sulfide (ZnS), PYROCERAM, RAYCERAM, sapphire and various glass-ceramics, yttria-based materials and others. At present, these materials are attached to the body of the missile using polymeric adhesives or epoxies. There is a need to develop hermetic joints between the advanced ceramic materials and the metallic elements comprising the body of the missile. These joints must be able to withstand temperatures in excess of 550.degree. C. for one minute or more and cyclic thermal environments from -40.degree. to 200.degree.. Existing polymeric materials cannot meet these requirements. Further, furnace heating of ceramic or metallic brazes to join the advanced IR or RF window components often results in degradation of the optical or microwave properties of the window.
Laser brazing offers the potential to reduce damage to the radome or infrared window during assembly by limiting the thermal degradation to regions adjacent to the joint. A focused laser beam provides a highly localized heat source necessary to form a metallurgical bond without affecting the bulk material.
Currently ceramic window or radome components can be joined to metals by mechanical fasteners, epoxy adhesives, or low-melting sealing glass. Higher temperature joints are possible using glass-ceramic materials or metallic braze alloys provided the inherent processing requirements can be met. The basic requirements for strong assemblies are chemical bonding and favorable stress gradients in the interfacial zone. This section describes conventional brazing and glass-ceramic sealing technology used for ceramic-to-metal joining.
The major problem of joining ceramic to metal is the difference in thermal expansion of the materials. In most cases, the metal to be joined has a higher thermal expansion than does the ceramic. Equally important in terms of joint reliability are the changes of coefficients due to composition gradients and microstructures that form because of the reactions at the interfaces. The ceramic and metallic components should be selected to minimize thermal expansion differences throughout their operating range. In general, the most reliable joints are those which form a thicker interfacial zone with a more extended and graded microstructure.
Brazing is uniquely suited to the fabrication of ceramic-to-metal joints and seals due to the introduction of a liquid phase at the interface which facilitates reactions, diffusion and stable chemical bonds. The conventional practice of brazing ceramics to metals involves a two step metallizing process in which a thin layer of metal is bonded to the ceramic component to improve the wettability of the ceramic surfaces by conventional low temperature filler metals. The metals are usually deposited by electroplating; however, in certain cases the coatings are produced by reducing oxides or sintered metal powder techniques.
The most widely known ceramic brazing process is the "molymanganese" process in which paint comprised of Mo and Mn powder is applied to the ceramic, generally Al.sub.2 O.sub.3 or BeO, and fired in a controlled hydrogen atmosphere to create a strongly adherent viscous melt composed of metals and residual oxides not completely reduced by sintering. This surface is usually plated with a 2 to 4 .mu.m thick layer of Ni or Cu, providing a surface which can be wetted easily by filler materials.
A more direct approach to ceramic/metal joining is the brazing of ceramics to metals using active filler metals. Because this is a one step process, it is simpler and more economical. In this process, a highly reactive metal is added to a normal braze alloy to promote chemical reaction and wetting at the ceramic interface. The most widely accepted alloy of this type is a silver-copper composition containing approximately 2% titanium, which is commercially available under the trade names CUSIL-ABA (GTE), LUCANEX 721 (Lucas-Milhaupt) and others. The high oxidation potential of titanium causes it to undergo a redox reaction with ceramic resulting in the spreading of the braze and formation of an oxide compound at the interface that is compatible with both phases, and produces a chemical bond at the interface. The alloy allows vacuum brazing in one step and can wet most metals without prior nickel plating of the substrate.
Another approach is the joining of a ceramic and a metal by an interlayer of glass. The normal procedure for achieving chemical bonding and favorable stress gradients in glass/metal joints is to preoxidize the metal and apply the glass in liquid phase. An example of this approach is the bonding of alumina to niobium in sodium vapor lamps. A glass filler consisting of Ca--Mg--Al.sub.2 O.sub.3 is placed in the joint, and the assembly is heated to approximately 1400.degree. C. to form a glass seal, primarily through the reaction of metal oxide (Al.sub.2 O.sub.3) with the liquid. The extent of the reaction and degree of crystallization are controlled by the heating schedule. Transformation of the glass into a high strength, multiphase glass-ceramic can occur during a carefully controlled high temperature furnace cycle consisting of three major segments: (1) seal segment flow of the glass and subsequent formation of a seal with the various metal substrates; (2) nucleation segment development of crystal nuclei in the glass; and (3) growth segment formation of crystals on nuclei, transforming the glass into a glass-ceramic.
The principal advantages of the glass-ceramic materials over conventional ceramics are associated with the absence of porosity in recrystallized materials to form a hermetic seal. The glass-ceramic material consists of a large portion of well-dispersed small crystals, generally smaller than 1 micron, along with a small amount of residual glass phase. The particular material properties can be programmed to a significant extent by a systematic variation of the chemistry and microstructure of the starting materials containing both glassy and crystalline phases, and by the selection of suitable crystallization process conditions. The coefficients of thermal expansion of these glass-ceramics are compatible with those of ceramic and metal over the temperature range of 77K to 1275K having a tailorable coefficient of thermal expansion ranging from 0.5 to 5.0.times.10.sup.-b /.degree.C.
The use of lasers for industrial processing of materials has increased steadily since the first successful operation of a laser in 1960. During the past decade in particular, the industrial laser industry has matured, and acceptance of lasers as viable production tools has become widespread. Laser processing of materials has generally been accomplished using industries CO.sub.2 lasers. The CO.sub.2 laser is the most powerful laser available and can produce power outputs in the multi-kilowatt range.
The interaction between a laser beam and the surface of a metal or ceramic is controlled primarily by the incident power density and the available interaction time. The laser-material interaction spectrum for high power industrial lasers is shown in FIG. 1. A particular combination of power density and interaction time defines a specific operational regime within the interaction spectrum. The laser brazing process would fall on the spectrum between the welding/cladding regime and the heat treatment regime (power density 10.sup.4 -10.sup.5 W/cm.sup.2, interaction time 0.1-1.0 sec.). The diagonal lines in FIG. 1 represent constant specific energy inputs which are computed as the product of the power density and the interaction time. Thus, it is not primarily the quantity of energy applied, but the power density and the rate at which the energy is applied, which produces the specific material processing effect desired. When the radiation intensity exceeds a critical value, the laser/material interactions are accompanied by a laser-induced plasma in front of the target surface. For continuous-wave CO.sub.2 laser irradiation of a metal, the threshold for plasma onset is about 10.sup.6 W/cm.sup.2. The formation of plasma should be avoided for laser brazing.
The coefficient of absorption (or reflection) for a given metal is affected by alloy concentration, surface roughness and degree of surface oxidation, and is also strongly temperature dependent. The amount of energy absorbed is also dependent upon the laser wavelength. Ferrous alloys typically have reflection coefficients of around 85% at room temperature. Fortunately the reflectivity decreases significantly at elevated temperatures. As the metal approaches its melt temperature, the absorption changes abruptly to near unity and additional energy coupling mechanisms take effect which are highly advantageous in cutting and welding applications.
The brazing process is best described as a form of heat treatment, since neither of the structural components in the joint is to be melted; only the filler material is raised above its melt temperature. Laser heat treatment processes have received a great deal of attention and have been successfully utilized in industrial applications of transformation hardening of ferrous alloys. In this process, a thin layer of the substrate is rapidly heated well into the austenitizing temperature by laser heating and subsequently cooled at a very fast rate, due to self quenching by conduction into the bulk part, to form a martensitic structure. The depth of hardening is closely determined by laser power, beam diameter and traverse speed.
Another factor affecting the joint thermal profile is the intensity distribution of the laser beam. Unlike laser welding or cutting, a wider beam with uniform energy distribution is desired to perform laser brazing. This can be accomplished by defocusing the laser beam or by altering the mode structure in the resonator. For example: changing the mode from the Gaussian distribution (TEM.sub.00) to a `donut` shaped mode (TEM01) will give more uniform heating. More elaborate distributions have been achieved using scanning mirrors and reflective optics. In any case, a power density of at least 10.sup.3 W/cm.sup.2 is usually needed.
The efficiency of laser heat treatment depends upon the absorption of laser energy by the work surface. Due to the reflectivity problems discussed earlier, some absorbent coatings are almost always used during laser heat treating. Most commonly used absorbent coatings include colloidal graphite, manganese phosphate, zinc phosphate and black paint. These coatings can increase absorptivity values to between 50% and 80%. Care must be taken to ensure that such additives do not contaminate the braze compositions.
The simplest approach to laser brazing is to heat the substrate or base metal adjacent to the joint and to rely upon thermal conductivity to heat the joint and filler metal to the brazing temperature. Under these conditions the controlling factors determining the temperature distribution are the geometry of the joint, the thermal conductivity of the materials and the coupling coefficient of the laser beam energy with the metal surface. Shining the beam directly on the filler materials is more problematic and often results in the braze materials being blown away by uncontrollable kinetic reactions in the interaction zone. However, this approach is necessary if glass-ceramic materials are used which have low thermal conductivity and a melt temperature above the metal component.
Most laser joining efforts have been directed toward fusion welding between similar or metallurgically compatible materials. The number of literature references to laser brazing or laser joining of ceramics is very limited. This is probably due to the innate tendency of ceramics to crack in the fused region during localized heating. However, supplementary heating and control of the heating and cooling cycles have enabled sound welds to be made in some ceramics. Joint strengths of up to 80 MPa (12 ksi) have been provided in CO.sub.2 laser-welded alumina up to 44 mm thick. A technique for laser-activated brazing of Si.sub.3 N.sub.4 ceramics using a CO.sub.2 laser beam for local heating and a mixture of refractory ceramic powders as brazing filler material has been reported. Again a furnace is used to preheat the part to 1100.degree. C. and the laser is used to elevate the braze to the brazing temperatures of 1680.degree. to 1850.degree. C. Laser brazing was accomplished in a very short time period (110 to 150 seconds), due to the high fluidity and reaction rate of the molten filter material. The narrow gap (3 to 5 .mu.m) butt-joint was filled with laser-activated molten braze by capillary attraction.
The above examples involve laser joining of ceramics to ceramics. Some work has also been reported on laser brazing of thin sections of metal using low heat input to minimize distortion. Both CO.sub.2 and Nd:YAG lasers were used to melt a variety of metal and powder filler materials including stainless steel, KOVAR, molybdenum and titanium. The best results were obtained using a low repetition rate pulsed Nd:YAG laser in the TEM.sub.01 mode with a defocused beam fired directly over the preplaced filler metal.